This invention relates to a pigment-coated phosphor for use in a color picture tube.
In a color TV receiver, it is of high importance to minimize the reflection of external light at the phosphor screen in order to enable the phosphor screen to display a picture image of good contrast under a bright external light. To minimize the reflection mentioned, it is customary to use a glass of low light transmittance for forming the front glass panel of a picture tube or to blacken the front glass panel except the portion where a phosphor layer is coated. However, a glass low in light transmittance fails to transmit sufficiently the light emitted from the phosphor layer, resulting in that the picture image displayed on the phosphor screen is darkened. The blackening method is not satisfactory, either, in that the reflection of the external light is not diminished sufficiently because a large portion of the front glass panel is coated with a phosphor layer and, thus, is not blackened.
It is known to the art to coat phosphors emitting red, green and blue lights with red, green and blue pigments, respectively, so as to decrease the reflection of color components of the external light other than the colors of the lights emitted from the phosphors. In a color picture tube utilizing pigment-coated phosphors, the degree of contrast depends to a large extent on the spectral reflectance of the pigment, rendering it very important to select suitable pigments. In addition to spectral reflectance, many properties must be taken into account for selecting suitable pigments. For example, the pigment should not contain harmful substances such as cadmium, lead and mercury. Further, the pigment should not be soluble in water because the phosphor in the form of slurry is coated on the front panel of a picture tube. Still further, heat resistance is of high importance because the slurry of phosphor coated on the glass panel is baked for burning off polyvinyl alcohol, etc. contained in the slurry. Naturally, the pigment should not melt and be free from changes in quality and color in the baking step.
Cadmium sulfoselenide, etc. are known as red pigments displaying brilliant red upon receipt of natural light. Certainly, pigments of this type are satisfactory in spectral reflectance and actually used in picture tubes in some cases. But, such pigments are poisonous and become discolored when heated. Thus, red oxide (Fe.sub.2 O.sub.3) is used in some cases in a color picture tube in place of the cadmium-based red pigment. However, red oxide is not satisfactory in spectral reflectance.
Under the circumstances, it is a matter of serious concern in this field to develop a pigment displaying a brilliant red, insoluble in water, harmless and having good heat resistance.
An object of this invention is to provide a red-emitting phosphor coated with a red pigment which is harmless and low in luminous reflectance. The pigment is higher in reflectance of light having 600 nm or more of wavelength and lower in reflectance of light having less than 600 nm of wavelength than conventional red pigments. Specifically the phosphor of this invention comprises a rare earth element-based phosphor body activated by europium and a red pigment coated on the phosphor body, said pigment consisting of indium sulfide or a combination of indium sulfide and an alkali metal sulfate.
Another object of this invention is to provide a method of producing a phosphor coated with a pigment, comprising coating a rare earth element-based phosphor body activated by europium with a complex compound containing indium sulfide and an alkali metal sulfide, followed by baking at 400.degree. to 600.degree. C. so as to convert the complex compound into a red pigment consisting of indium sulfide and an alkali metal sulfate.